In order to better evaluate the performance of rotating machines, it has been found desirable to represent the performance characteristics of rotating machines in terms of shaft position, or shaft phase angle instead of in terms of uniform time intervals. Unfortunately, performance data for rotating machines is generally produced and sampled at uniform time intervals. In order to produce shaft position or phase angle based data, the uniform time interval data must be processed in a manner that converts the time based data to phase angle based data.
The classical approach used in the prior art to convert time based performance data to uniform shaft phase angle data utilizes numerous hardware components to accomplish the conversion. Such hardware may include: a sensor that monitors the rotation of a shaft; a counter that registers pulses produced when the shaft is rotated; one or more tracking analog filters that limit the aliasing errors associated with data sampling and, a tracking ratio synthesizer that generates sampling pulses synchronously with the rotation of the shaft, whose velocity is arbitrary with respect to time. Because the tracking analog antialiasing filters must communicate with the tracking ratio synthesizer and adjust for varying shaft velocity, they are complex and, therefore, expensive to design.
Tracking ratio synthesizers are undesirable because of their inherent time delay. More specifically, a tracking ratio synthesizer includes an oscillator that generates the synchronized sampling pulses and a feedback control loop, such as a phase lock loop circuit, to control the oscillator frequency. Because the feedback control loop responds to changes in shaft velocities after a change in shaft velocity is sensed by the control loop, a time delay occurs between changes in shaft velocity and the synchronized output of the tracking ratio synthesizer. As a result of this time delay, the monitoring hardware used by the prior art lags behind the actual machine condition and may be sampling data at a rate that does not agree with the current shaft velocity. When the oscillator frequency drifts away from the preselected sampling frequency the control loop will correct the error; however, during this correction period, errors are introduced into the system which contaminate and distort the input data being measured. Similarly, when the feedback control loop attempts to correct the oscillator frequency, it may overshoot the target frequency and cause additional system error, which may in turn cause a further time delay before the frequency is fully corrected.
This invention is directed to overcoming the foregoing disadvantages. More particularly, this invention provides a method and apparatus for monitoring the performance of rotating machines in terms of shaft position or phase angle that does not require numerous complex and expensive hardware components. The invention also eliminates the time delay associated with such hardware and reduces the error signals introduced into the system, thereby resulting in a more cost effective, more responsive and more accurate method for monitoring the performance of rotating machines.